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ISSN 2047-0371

Surf Zone Hydrodynamics: Measuring Waves and Currents

Kris W. Inch1 1 Coastal Processes Research Group, School of Marine Science and Engineering, University of Plymouth ([email protected])

ABSTRACT: Wave breaking is the most significant energy input into wave-dominated coastal environments and is responsible for driving a number of interrelated, hydrodynamic processes in a region known as the surf zone. Surf zone hydrodynamics occur over a range of spatial and temporal scales and are constantly changing. As a result, obtaining measurements of waves and currents in the surf zone can be challenging and field experiments require detailed planning to ensure the collection of useful data. Measurements of surf zone hydrodynamics are divided into two general categories: (1) non-directional wave measurements; and (2) directional wave and measurements. Near-bed pressure transducers are by far the most common method of measuring the non-directional properties of waves. Acoustic sensors are being used with increasing frequency for Eulerian measurements of surf zone currents, whereas Lagrangian surf zone drifters are most useful for measuring flow patterns in rip currents, surf zone circulation and alongshore currents. This article gives a summary of the various methods of measuring waves and currents in the surf zone. Additionally, simple data analysis techniques to study infragravity waves are demonstrated. KEYWORDS: surf zone, breaking waves, pressure transducer, current meters, infragravity waves.

Introduction in determining the breaker type and dissipative characteristics of the surf zone. The surf zone is that part of the shoreface Three types of breaking waves are commonly extending from the seaward boundary of recognised (Figure 1); spilling, plunging and wave breaking to the zone; the part of surging (Galvin, 1968). Spilling waves the that is alternately covered and typically occur on low gradient and exposed by wave uprush and backwash. dissipate their energy gradually over a wide Breaking waves drive a number of surf zone. With plunging breakers, the interrelated surf zone processes such as the shoreward face of the wave steepens until it creation of turbulent bores, wave set-up, is vertical and the crest curls over and nearshore currents and low frequency plunges forward and downward as an intact motions. Together these processes force the mass of water. Plunging waves are more entrainment and transport of sediment, which energetic than spilling waves at the point of leads to morphological change. For a breaking and are normally associated with comprehensive overview of surf zone narrower surf zones and steeper beaches. processes see Komar (1998), or the Surging waves are found on steep, reflective introductory texts of Aagaard and Masselink beaches where there is often no clear surf (1999), Woodroffe (2002), Davidson-Arnott zone as the waves slide up the beach without (2010) and Masselink et al. (2011) for a more physically breaking. In reality, there is a succinct summary. continuum of breaker types blending from one to another and on a natural beach with a The width and characteristics of the surf zone spectrum of wave heights and periods it is vary constantly, driven by changes in the common to see a range of breaker types at a elevation, incident and direction, given time. A number of dimensionless low-frequency motions and local wind speed. parameters have been developed to predict The beach slope also plays an important role

British Society for Geomorphology Geomorphological Techniques, Chap. 3, Sec. 2.3 (2014) Surf Zone Hydrodynamics: Measuring Waves and Currents 2 the breaker type and surf zone state. The break close to creating a narrow surf most widely used of these is the Iribarren zone that can remain unsaturated. number � defined as ���� The inner surf zone is of particular � = importance as the conditions here force the �! �! hydro and sediment dynamics in the swash where � is the beach slope, � is the breaker zone, which is arguably the most dynamic ! part of the nearshore region (Masselink and height and �! is the deep water wavelength given by linear wave theory. Spilling waves Puleo, 2006). Water motion in the inner surf zone is typically dominated by infragravity occur when � < 0.4, plunging waves occur waves, particularly on dissipative beaches. when 0.4 < � < 2.0, and surging waves occur Infragravity waves are low-frequency (0.005- when � > 2.0 (Battjes, 1974). 0.05 Hz) waves that approach the shoreline bound to incident wave groups but become free waves in the surf zone following incident wave breaking. Infragravity waves are an important research topic for coastal scientists as they play an important role in beach and erosion, especially during storms (Russell, 1993). Due to their long wavelengths, which prevent breaking, infragravity waves reflect from the shoreline and travel seawards giving rise to a cross- shore quasi-standing wave pattern (Guza and Thornton, 1985). Infragravity energy in the surf zone increases with increasing offshore wave height; however, recent studies have revealed that under very energetic wave conditions infragravity waves may also become saturated (e.g. Senechal et al., 2011a; Guedes et al., 2013; De Bakker et al., 2014).

In addition to wave motion, three types of

quasi-steady, wave-induced currents exist in Figure 1: The three types of . the surf zone; bed return flow, alongshore Note the different break point locations in currents and rip currents. These currents relation to the shoreline. Modified from exist simultaneously and are driven by cross- Davidson-Arnott (2010). shore and alongshore gradients in the mean water level caused by variations in the wave In the inner surf zone of low gradient, breaker height. The intensity of nearshore dissipative beaches, the incident wave height currents increases with increasing incident � is controlled by the local water depth wave height. Thus, the strongest currents (Thornton and Guza, 1982) and can be capable of transporting vast quantities of expressed by sediment occur during storms (Senechal et � = � ℎ al., 2011b). Additionally, rip currents pose a significant hazard to water users and are a where � is a coefficient ranging from 0.3-0.6 major cause of lifeguard rescues around the and increases with beach slope, and ℎ is world (Short and Brander, 1999; Scott et al., water depth. When the wave height is limited 2007, 2008). by the local water depth the surf zone is considered to be saturated. Under saturated Measurements of surf zone processes are of conditions, to maintain a constant relationship vital importance for estimating potential storm between wave height and water depth, an damage, modelling shoreline evolution and increase in offshore wave height acts only to designing shoreline management plans. increase the width of the surf zone. However, Waves and currents are well documented in on steep, sandy beaches plunging waves the coastal literature; advances in instrument

British Society for Geomorphology Geomorphological Techniques, Chap. 3, Sec. 2.3 (2014) 3 Inch, K.W. technology and analysis techniques have Nowadays, almost all surf zone field improved our knowledge considerably over experiments use bottom-mounted pressure the last few decades. However, certain transducers. Pressure transducers measure aspects remain poorly understood, such as pressure variations associated with passing surf zone hydrodynamics during extreme waves above and this pressure is converted storms. The purpose of this entry is to (either by the sensor or in post-processing) provide an overview of the current in situ into the equivalent water depth. Pressure methods of measuring surf zone waves and transducers are most commonly attached to currents, and to give a simple demonstration a temporary frame driven into the of wave data analysis. (Figure 2a), but can also be fixed to permanent features such as shore platforms

or buried beneath the surface of the beach. Surf Zone Measurements An advantage of using buried pressure transducers is to avoid corruption of the For convenience, the following section on signal caused by dynamic pressure variations surf zone measurements is split into two main from accelerating and decelerating flows categories: (1) non-directional wave (Austin et al., 2014). Unlike wave staffs, measurement; and (2) directional wave and pressure transducers are not directly current measurement. Non-directional wave exposed to wave action. They are also measurements are measurements of the relatively easy to deploy, can be co-located water surface elevation from which with other sensors and are often self-logging information about wave height and period can eliminating the need for cables. be obtained. Current measurements are measurements of the velocity vector and are The main limitation of using pressure used to study nearshore currents and the transducers is depth attenuation of the directional properties of waves. pressure signal. The extent of depth attenuation is frequency dependant; as depth Non-directional wave measurement increases high frequency signals are lost before low frequency signals. Depth Field measurements of waves have attenuation can be corrected for during post- previously been collected using a form of processing by applying a frequency surface piercing wave staff (among others, dependant depth attenuation factor �(�) Thornton and Guza, 1983; Davidson-Arnott derived from linear wave theory as: and McDonald, 1989). Wave staffs are part of cosh �� an electronic circuit that takes advantage of � � = the conductivity of water by recording the cosh �ℎ change in electrical resistance or capacitance where � is the frequency dependant wave as waves pass the instrument. These number (2�/�, where � is wavelength) and � recordings are then converted to is the height of the pressure sensor above measurements of water depth using the bed. A consequence of using this method calibrated signals from known water depths. to correct for depth attenuation is that it The sensors can be attached to a temporary introduces high frequency noise, which can support driven into the sand or attached to a cause significant error at the data analysis more permanent structure such as a pier. stage. Therefore a high frequency cut-off The advantage of using wave staffs is that should be chosen beyond which energy in they provide a direct measure of the sea the wave spectrum (see Wave Data Analysis) surface elevation. The main disadvantage of is discarded. The high frequency cut-off wave staffs is that they are exposed to wave should be chosen carefully depending on action when deployed in shallow water or in factors such as the sensor depth and local energetic surf zones. This can damage wave wave climate. However, an alternative staffs or displace them from their intended method of determining the high frequency vertical positions, thus causing error in the cut-off � is by water depth measurements. Furthermore, ! mounting requirements restrict the use of �! = 0.564� (� �) wave staffs at certain locations and may limit the deployment of additional sensors at the (Green, 1999; Aagaard et al., 2002) where � same location. is radian frequency ( 2�� , where � is frequency), � is gravitational acceleration

British Society for Geomorphology Geomorphological Techniques, Chap. 3, Sec. 2.3 (2014) Surf Zone Hydrodynamics: Measuring Waves and Currents 4 (9.81 m s-2) and � is the depth of the providing continuous measurements of bed pressure sensor. Pressure transducers level elevation (e.g. Ridd, 1992; Saulter et al., deployed in the surf zone tend to be located 2003; Arnaud et al., 2009; Puleo et al., 2010; in relatively shallow water compared to open et al., 2014). Whilst the unknown height of deployments. Nonetheless, correcting the pressure transducer can cause error in for depth attenuation is an important process, measurements of total water depth, Ruessink especially in macrotidal regions and low fetch (1999) suggests that the impact of these environments where high frequency wind uncertainties on wave height calculations is < waves prevail. In addition to depth 10%. attenuation, there is also the need to correct for atmospheric pressure. Some pressure Directional wave and current transducers do this automatically using the mean atmospheric pressure at measurement (10.1325 dbar); however, this does not Wave staffs and pressure transducers account for local variations in atmospheric provide a time series of the water surface pressure or short term fluctuations during an elevation, which is useful for studying wave experiment, for which local pressure height and period. However, in order to observations are required. investigate directional wave properties and nearshore currents, the horizontal component An additional source of error in water depth of water motion also needs to be measured. measurements provided by pressure Current data can broadly be divided into two transducers arises from the unknown height categories: (1) Eulerian observations; and (2) of the sensor above the bed. If this offset is Lagrangian observations. Eulerian methods measured at the start of an experiment and at measure fluid flow at a fixed location through every opportunity thereafter, a linear function time, whereas Lagrangian methods follow may be used to correct for bed level change fluid parcels through space and time. between measurements. However, the Eulerian observations are collected by in situ results of recent studies in the inner surf zone current meters, often co-located with (e.g. Puleo et al., 2014) suggest that net bed pressure transducers and referred to as PUV level change over a tidal cycle may not be set-ups (Figure 2b) due to the three linear but rather the result of just a few “large” quantities measured; pressure (P) with cross- waves. A way to overcome this is by co- shore (U) and alongshore (V) velocities locating the pressure transducer with a (Morang et al., 1997). sensor, such as an ultrasonic distance meter,

Figure 2: Examples of (a) a bottom-mounted pressure transducer, (b) a PUV rig equipped with an acoustic Doppler velocimeter, and (c) surf zone drifters. (Photos: K. Inch and E. Woodward)

British Society for Geomorphology Geomorphological Techniques, Chap. 3, Sec. 2.3 (2014) 5 Inch, K.W. These types of measurements are useful for Doppler profilers (ADPs) to monitor surf zone studying directional wave properties and hydrodynamics (e.g. Senechal et al., 2011b). near-bed currents. Lagrangian observations ADPs operate using the same basic are most useful in studies of rip currents, surf principles as ADVs and are therefore also zone circulation and surf zone flushing where highly sensitive to bubbles and sediment. current magnitude and direction vary spatially However, by measuring the return signal in in the surf zone. much smaller time increments, ADPs determine the velocity vector for a series of discrete “bins” over a portion of the water Eulerian observations column. This makes ADPs especially useful Since their introduction in the 1970s, for studying boundary layer dynamics in the electromagnetic current meters (EMCMs) surf zone. For example, Puleo et al. (2012) have been widely used to measure currents used an ADP to investigate the inner surf in the surf zone. EMCMs measure the zone boundary layer at a spatial resolution of voltage generated by waves passing through 1 mm and a temporal resolution of 100 Hz on a fluctuating magnetic field produced by the a microtidal beach in Florida, USA. sensor. Faraday’s law is then used to convert the voltage into the proportional current Lagrangian observations velocity along two perpendicular axes (u,v). Over the past decade or so, acoustic Doppler A Lagrangian method which is particularly velocimeters (ADVs) have become a popular popular in Australian research is releasing a alternative to EMCMs. ADVs operate by bright, inert dye into the surf zone and emitting acoustic signals into the water tracking its movement (e.g. Huntley et al., column and recording the return signal 1988; Brander, 1999; Brander and Short, backscattered by fine material a short 2001). Dye tracking is useful in providing distance from the sensor. Current velocity is qualitative data on the location and path of calculated using the phase lag between nearshore currents and is a valuable tool in successive return signals. ADVs are seen as beach safety education. However, it does not an advancement on EMCMs as they provide any data on actual current velocities. measure three-dimensional (u,v,w) velocity An increasingly popular technique for components and are non-intrusive (MacVicar measuring the flow patterns in nearshore et al., 2007). Also, EMCMs are prone to error currents is using GPS-tracked surf zone if the sensor is located too close to the drifters (Figure 2c). Surf zone drifters are or free surface, drift of the zero offset buoyant, PVC tubes equipped with on-board and electronic interference (Guza et al., GPS data loggers which record the drifter’s 1988). A significant problem with using ADVs position in the surf zone at typically 0.5-1 Hz in the surf zone, however, is their high (Schmidt et al., 2003). The drifters are sensitivity to bubbles and sediment which equipped with a damping plate designed to results in poor data (determined by the signal allow broken waves to pass without rapidly to noise ratio) being removed in quality transporting the drifter onshore (Schmidt et control procedures (Elgar et al., 2005; al., 2003). Accuracies of < 0.4 m in position Feddersen, 2010). The lower sensitivity of and < 0.01 m s-1 in velocity can be achieved EMCMs means that they remain a practical by post-processing the raw GPS data from a and much used instrument in surf zone static base position (MacMahan et al., 2009). studies and, under highly turbulent The combined tracks of multiple drifters conditions, may outperform ADVs (e.g. provide detailed information on mean Rodriguez et al., 1999). Elgar et al. (2001) nearshore flow patterns and velocities (e.g. simultaneously deployed EMCMs and ADVs Austin et al., 2010, 2014). Surf zone drifters on the same frame in the surf zone to provide have been observed to closely follow a direct and detailed comparison of their simultaneous dye releases as well as provide performance. They found that the data were velocity measurements in good agreement highly correlated in velocities up to 3 m s-1, with those from in situ current meters, thus confirming the ability of both instruments to confirming their ability to make valid perform well at measuring surf zone currents. Lagrangian observations of surf zone currents (Schmidt et al., 2003; Johnson and There has been increasing interest in recent Pattiaratchi, 2004; MacMahan et al., 2009). A years in using pulse-coherent acoustic recent study by McCarroll et al. (2014) used

British Society for Geomorphology Geomorphological Techniques, Chap. 3, Sec. 2.3 (2014) Surf Zone Hydrodynamics: Measuring Waves and Currents 6 34 surf zone drifters to investigate imagery (e.g. Holman et al., 2006; Holman behaviour on an embayed beach in New and Stanley, 2007; De Vries et al., 2011), X- South Wales, Australia. They were able to band radar (e.g. Ruessink et al., 2002; make 293 individual drifter deployments over Catalan et al., 2014; Haller et al., 2014) and a single ebbing tidal cycle, thus providing a LIDAR (e.g. Blenkinsopp et al., 2012). detailed observation of the nearshore flow Possible advantages of using remote sensing patterns (Figure 3). methods include better synoptic measurements, lower maintenance costs, improved robustness and longer deployments Remote sensing including during storms. A potential It is beyond the scope of this entry to discuss disadvantage of remote sensing is that remote sensing methods in detail; however, it measurements are often based on a proxy should be acknowledged that remote sensing rather than a direct measurement; therefore, techniques are being used with increasing the accuracy of the measurements depends frequency to study nearshore waves and on how well the proxy represents the variable currents (for a review, see Holman and being studied. Haller, 2013). These methods include video

Figure 3: Drifter tracks coloured by velocity magnitude (a), and mean velocity magnitude and direction determined from individual drifter observations (b). Contours are at 0.5 m spacing and the edge of the surf zone is indicated by the black dashed line in (a) and white dashed line in (b). (Source: McCarroll et al., 2014)

Wave Data Analysis research and the type of data available. In this section, the analysis of wave and current The analysis of surf zone data is very data to investigate infragravity waves in the complex and it is far beyond the scope of this surf zone (see Introduction) is discussed as article to discuss all types of analysis an example. Data collected from the inner techniques. The most appropriate analysis surf zone of Perranporth Beach, Cornwall, procedure depends on the aim of the UK, is used to demonstrate the analysis

British Society for Geomorphology Geomorphological Techniques, Chap. 3, Sec. 2.3 (2014) 7 Inch, K.W. techniques. Perranporth is a macrotidal, Time domain analysis dissipative beach with a mean of The irregular nature of natural waves is 5.4 m and is composed of medium sand. evident in the example time series of water Observations of pressure and velocity (u,v,w) surface elevation and cross-shore velocity were logged continuously at 4 Hz using a co- shown in Figure 4a and 4b. The two time located pressure transducer and ADV during a large storm in October 2013. The series have limited value in their raw state and are best described quantitatively using instruments were mounted onto a temporary statistics. The two most common statistical scaffold frame driven into the sand in the parameters used to describe a wave time (Figure 2b). The height of the instruments above the bed was measured series are the �! (also before and after each tide and a linear �! !) which is the mean of the highest one function was used to correct for changes in third of waves, and the peak wave period �! bed level. The pressure data were converted which is the wave period associated with the to water surface elevation, with a depth maximum wave energy derived from the correction using linear wave theory and a wave spectrum (described below). Other high frequency cut-off of 1 Hz (see Surf Zone commonly used wave parameters include the Measurements), and detrended prior to mean wave height �, maximum wave height analysis. �!"# and zero-crossing wave period �! . A summary of the various wave parameters There are generally two approaches to that can be derived from both time and analysing wave data; analysis in the time frequency domain analysis is given by domain and in the frequency domain. Morang et al. (1997).

Figure 4: Raw time series (5 mins) of water surface elevation � (a) and cross-shore velocity u (b) collected in the inner surf zone of Perranporth Beach during storm conditions with an offshore significant wave height of 4.31 m. Low pass filtered (0.005-0.05 Hz) water surface elevation (c) and cross-shore velocity (d) time series (red) plotted with the original time series (black).

British Society for Geomorphology Geomorphological Techniques, Chap. 3, Sec. 2.3 (2014) Surf Zone Hydrodynamics: Measuring Waves and Currents 8 One method to obtain these wave its incoming and outgoing components and parameters is through wave-by-wave corresponding significant wave heights were analysis using the zero-downcrossing calculated. The significant wave height of the method. The zero-downcrossing method outgoing infragravity time series was 0.23 m, defines an individual wave by two successive indicating that some wave energy was indeed downward crossings of the mean water level reflected. It is also possible to separate the by the water surface elevation. Alternatively, incoming and outgoing wave components in Longuet-Higgins (1952) proposed a method the frequency domain (discussed below). based on the Rayleigh distribution, which calculates the various wave height parameters using the standard deviation of Frequency domain analysis the water surface elevation time series. Wave data analysis in the frequency domain Significant wave height, for example, can be is achieved through spectral analysis. estimated from Spectral analysis is based on the fast Fourier transformation, which assumes that a time � = 4� ! ! series is composed of a finite number of where �! is the standard deviation of the sinusoids at discrete frequencies. Spectral water surface elevation. To calculate wave analysis partitions a time series into its statistics over particular frequency ranges constituent parts and produces an auto- (e.g. infragravity and incident), high- and low- spectrum (sometimes referred to as a wave pass filters can be applied to the original time spectrum); a plot of wave variance series. Figure 4c and 4d shows the filtered (proportional to wave energy) as a function of infragravity components plotted with the frequency. An outline and worked example of original time series from Figure 4a and 4b. the various steps involved to produce a wave The infragravity time series follow closely the spectrum is given by Hegge and Masselink original time series for both water surface (1996). Figure 5a-c shows the water surface elevation and cross-shore velocity. This is a elevation, cross-shore and alongshore clear indication that the dominant water velocity auto-spectra of an extended version motion is at infragravity frequencies and the of the time series in Figure 4. The auto- calculated significant wave heights reflect spectra reveal that most of the variance in this; 0.58 m and 0.30 m for the infragravity water surface elevation and cross-shore and incident bands respectively. This is velocity is at infragravity frequencies; 87.7% typical of water motion in the inner surf zone and 88.4% respectively. The alongshore of dissipative beaches. velocity auto-spectrum, however, shows less variance over a broader range of frequencies. As mentioned in the introduction, the long wavelengths of infragravity waves inhibit An extension of spectral analysis that is wave breaking allowing some energy to useful in the study of infragravity waves is reflect from the shoreline. Therefore, the low cross-spectral analysis. Cross-spectral frequency signal of a wave time series will analysis is used to determine the level of co- comprise both incoming (shoreward) and variance and phase lag between two time outgoing (seaward) components. Guza et al. series (Jenkins and Watts, 1968). If the (1984) proposed a method to decompose the cross-shore structure of infragravity waves is shoreward and seaward propagating wave standing, cross-spectral analysis will reveal a signals in the time domain using the water phase difference of 90° (�/2) between the surface elevation and cross-shore velocity by: water surface elevation and cross-shore velocity (Suhayda, 1974). Cross-spectral � + ℎ � � analysis has also been used to investigate � = !" 2 the relationship between infragravity waves and wave groups in the surf zone (e.g. � − ℎ � � Masselink, 1995). �!"# = 2 where �!" and �!"# are the incoming and The decomposition of wave energy into outgoing wave components, � is water incoming and outgoing components in the surface elevation and � is cross-shore frequency domain can be achieved in two velocity. This method was used to separate ways; (1) using pressure and cross-shore the signal in Figure 4c into velocity measurements from co-located

British Society for Geomorphology Geomorphological Techniques, Chap. 3, Sec. 2.3 (2014) 9 Inch, K.W. sensors, or (2) using pressure measurements decomposition method used will largely only from a cross-shore array of pressure depend on instrument availability and the transducers. The method of Sheremet et al. overall aim of the study. De Bakker et al. (2002) is an example of the first technique (2014) performed a comparison between the where the incoming and outgoing energy � at � − � method of Sheremet et al. (2002) and each discrete frequency is calculated as: the array method of Van Dongeren et al. (2007) using data collected in the surf zone of 1 ℎ ℎ a dissipative beach in the Netherlands and �± � = � � + � (�) ± 2 � (�) 4 ! � ! � !" found relatively good agreement between the two methods. where � and � are the pressure and cross- ! ! Following the decomposition of wave energy shore velocity auto-spectra respectively and by one of the above methods, frequency � is the � − � cospectrum. The second !" dependant (and bulk) reflection coefficients technique uses calculations of wave celerity �!(�) can be calculated which are simply the and phase lag as waves travel through an ratio of seaward to shoreward propagating array of normally three or more pressures wave energy flux �±(�) (calculated as transducers. A number of solutions exist that ± employ this method such as those of Gaillard � (�) �ℎ). et al. (1980), Battjes et al. (2004) and Van Dongeren et al. (2007). The type of

Figure 5: Auto-spectra of water surface elevation � (a), cross-shore velocity u (b) and alongshore velocity v (c). Incoming and outgoing water surface elevation auto-spectra (d) using the separation method of Sheremet et al. (2002) and corresponding reflection coefficients R2 (e). Normalised spectral density S versus distance offshore for three infragravity frequencies (f-h). Vertical dotted lines in a-e indicate the infragravity-incident frequency transition of 0.05 Hz.

British Society for Geomorphology Geomorphological Techniques, Chap. 3, Sec. 2.3 (2014) Surf Zone Hydrodynamics: Measuring Waves and Currents 10 Figure 5e shows the frequency dependant remain a valuable tool, especially during reflection coefficients corresponding to the turbulent conditions. incoming and outgoing spectra in Figure 5d (estimated using the method of Sheremet et Detailed planning, including a thorough al. (2002) outlined above). The reflection understanding of the various sensors and coefficients show that only frequencies at the analysis procedures, is crucial to ensure the lower end of the infragravity band reflect a collection of a high quality, useful dataset. significant amount of energy; �! is less than There are several important considerations 0.1 for all frequencies higher than 0.0273 Hz that need to be included such as the local indicative of > 90% energy dissipation. wave climate, sampling strategy, data retrieval and analysis techniques, and the To gain a detailed insight into the cross-shore overall aim of the research. Researchers are structure and transformation of infragravity advised to review the relevant literature (and incident) waves, the techniques before undertaking a field experiment in order described above can be applied to data to achieve the optimal set-up for their project. collected at a number of cross-shore locations. This can be achieved through the deployment of an instrument array, or Acknowledgements deploying a single instrument rig and using the tide as a surrogate for changing cross- The comments and suggestions of two shore position. The latter option requires anonymous reviewers helped to improve the fewer instruments, but is limited to sites with clarity of this article. The author also thanks a large tidal range, linear beach profile and Dr Mark Davidson and Prof. Paul Russell for no change in forcing conditions during the their constructive feedback on an early draft. study. The cross-shore structure of energy at Fieldwork assistance at Perranporth was three discrete frequencies in the infragravity provided by Dr Mark Davidson, Prof. Paul band is demonstrated in Figure 5f-h by Russell, Dr Tim Scott and Mr Luis Melo de plotting the spectral density at these Almeida. frequencies versus distance offshore. In this example the tide was used as a proxy for changing cross-shore position, hence the References spectral density is normalised by the offshore Aagaard T, Masselink G. 1999. The Surf wave height. It can be seen that at 0.0078 Hz Zone. In Handbook of Beach and Shoreface there is a clear (anti)nodal structure symbolic Morphodynamcis, Short AD (ed.). Wiley: of standing waves with antinodes at 15 m and Chichester; 72-118. 40 m and a node at 24 m. This is partially lost at 0.0234 Hz and completely absent at Aagaard T, Black KP, Greenwood B. 2002. 0.0430 Hz which displays a progressive wave Cross-shore suspended sediment transport in pattern. This agrees well with the reflection the surf zone: a field-based parameterization. coefficients in Figure 5e. 185: 283-302. Arnaud G, Mory M, Abadie S, Cassen M. 2009. Use of a resistive rods network to Concluding Remarks monitor bathymetric evolution in the The surf zone is highly energetic and can be surf/swash zone. Journal of Coastal a challenging environment in which to collect Research SI 56: 1781-1785. wave and current measurements. The data, Austin M, Scott T, Brown J, Brown J, however, are invaluable and there have been MacMahan J, Masselink G, Russell P. 2010. many advances in instrument technology Temporal observations of rip current over the last few decades, which have helped circulation on a macro-tidal beach. to further our knowledge of surf zone Research 30: 1149-1165. hydrodynamics. Pressure transducers are Austin MJ, Masselink G, Scott TM, Russell undeniably the most commonly used and PE. 2014. Water-level controls on macro-tidal arguably the most valuable device for rip currents. Continental Shelf Research 75: measuring waves. There has been a shift 28-40. towards the use of acoustic sensors for measuring surf zone currents, yet EMCMs

British Society for Geomorphology Geomorphological Techniques, Chap. 3, Sec. 2.3 (2014) 11 Inch, K.W. Battjes JA. 1974. Surf similarity. Proceedings Dissipation Rate. Journal of Atmospheric and of the 14th Conference, Oceanic Technology 27: 2039-2055. ASCE, 466-480. Gaillard P, Gauthier M, Holly F. 1980. Method Battjes JA, Bakkenes HJ, Janssen TT, Van of analysis of random wave experiments with Dongeren AR. 2004. Shoaling of reflecting coastal structures. Proceedings of subharmonic gravity waves. Journal of the 17th International Conference on Coastal Geophysical Research 109: C02009. Engineering, ASCE, 204-220. Blenkinsopp CE, Turner IL, Allis MJ, Peirson Galvin CJ. 1968. Breaker type classification WL, Garden LE. 2012. Application of LIDAR on three laboratory beaches. Journal of technology for measurement of time-varying Geophysical Research 73: 3651-3659. free-surface profiles in a laboratory wave Green MO. 1999. Test of sediment initial- flume. Coastal Engineering 68: 1-5. motion theories using irregular-wave field Brander RW. 1999. Field observations on the data. Sedimentology 46: 427-441. morphodynamic evolution of a low-energy rip Guedes RMC, Bryan KR, Coco G. 2013. current system. Marine Geology 157: 199- Observations of wave energy fluxes and 217. swash motions on a low-sloping, dissipative Brander RW, Short AD. 2001. Flow beach. Journal of Geophysical Research 118: Kinematics of Low-energy Rip Current 3651-3669. Systems. Journal of Coastal Research 17: Guza RT, Thornton EB, Holman RA. 1984. 468-481. Swash on steep and shallow beaches. Catalan PA, Haller MC, Plant WJ. 2014. Proceedings of the 19th International Microwave backscattering from surf zone Conference on Coastal Engineering, ASCE, waves. Journal of Geophysical Research 708-723. 119: 3098-3120. Guza RT, Thornton EB. 1985. Observations Davidson-Arnott RGD, McDonald RA. 1989. of Surf Beat. Journal of Geophysical Nearshore water motion and mean flows in a Research 90: 3161-3172. multiple parallel bar system. Marine Geology Guza RT, Clifton MC, Rezvani F. 1988. Field 86: 321-338. Intercomparisons of Electromagnetic Current Davidson-Arnott R. 2010. Introduction to Meters. Journal of Geophysical Research 93: Coastal Processes and Geomorphology. 9302-9314. Cambridge University Press: New York. Haller MC, Honegger D, Catalan PA. 2014. De Bakker ATM, Tissier MFS, Ruessink BG. Rip Current Observations via Marine Radar. 2014. Shoreline dissipation of infragravity Journal of Waterway, Port, Coastal and waves. Continental Shelf Research 72: 73- Ocean Engineering 140: 115-124. 82. Hegge BJ, Masselink G. 1996. Spectral De Vries S, Hill DF, De Schipper MA, Stive Analysis of Geomorphic Time series: Auto- MJF. 2011. Remote sensing of surf zone Spectrum. Earth Surface Processes and waves using stereo imaging. Coastal 21: 1021-1040. Engineering 58: 239-250. Holman RA, Symonds G, Thornton EB, Elgar S, Raubenheimer B, Guza RT. 2001. Ranasinghe R. 2006. Rip spacing and Current Meter Performance in the Surf Zone. persistence on an embayed beach. Journal of Journal of Atmospheric and Oceanic Geophysical Research 111: C01006. Technology 18: 1735-1746. Holman RA, Stanley L. 2007. The history and Elgar S, Raubenheimer B, Guza RT. 2005. technical capabilities of Argus. Coastal Quality control of acoustic Doppler Engineering 54: 477-491. velocimeter data in the surfzone. Holman RA, Haller MC. 2013. Remote Measurement Science and Technology 16: Sensing of the Nearshore. Annual Review of 1889-1893. Marine Science 5: 95-113. Feddersen F. 2010. Quality Controlling Surf Huntley DA, Hendry MD, Haines J, Zone Acoustic Doppler Velocimeter Greenidge B. 1988. Waves and Rip Currents Observations to Estimate the Turbulent

British Society for Geomorphology Geomorphological Techniques, Chap. 3, Sec. 2.3 (2014) Surf Zone Hydrodynamics: Measuring Waves and Currents 12 on a Caribbean , Jamaica. stress and friction on the foreshore of a Journal of Coastal Research 4: 69-79. microtidal beach. Coastal Engineering 68: 6- 16. Jenkins GM, Watts DG. 1968. Spectral Analysis and its Applications. Holden-Day: Puleo JA, Lanckriet T, Blenkinsopp C. 2014. San Francisco. Bed level fluctuations in the inner surf and swash zone of a dissipative beach. Marine Johnson D, Pattiaratchi C. 2004. Application, Geology 349: 99-112. modelling and validation of surfzone drifters. Coastal Engineering 51: 455-471. Ridd PV. 1992. A sediment level sensor for erosion and siltation detection. Estuarine, Komar PD. 1998. Beach Processes and Coastal and Shelf Science 35: 353-362. Sedimentation: Second Edition. Prentice Hall: New Jersey. Rodriguez A, Sanchez-Arcilla A, Redondo JM, Mosso C. 1999. Macroturbulence Longuet-Higgins MS. 1952. On the statistical measurements with electromagnetic and distribution of the height of sea waves. ultrasonic sensors: a comparison under high- Journal of Marine Research 11: 245-266. turbulent flows. Experiments in Fluids 27: 31- MacMahan J, Brown J, Thornton E. 2009. 42. Low-Cost Handheld Global Positioning Ruessink BG. 1999. Data report 2.5D System for Measuring Surf-Zone Currents. experiment Egmond aan Zee. Department of Journal of Coastal Research 25: 744-754. Physical Geography, University of Utrecht, MacVicar BJ, Beaulieu E, Champagne V, The Netherlands. Roy AG. 2007. Measuring water velocity in Ruessink BG, Bell PS, Van Enckevort IMJ, highly turbulent flows: field tests of an Aarninkhor SGJ. 2002. Nearshore bar crest electromagnetic current meter (ECM) and an location quantified from time-averaged X- acoustic Doppler velocimeter (ADV). Earth band radar images. Coastal Engineering 45: Surface Processes and Landforms 32: 1412- 19-32. 1432. Russell PE. 1993. Mechanisms for beach Masselink G. 1995. Group bound long waves erosion during storms. Continental Shelf as a source of infragravity energy in the surf Research 13: 1243-1265. zone. Continental Shelf Research 15: 1525- 1547. Saulter AN, Russell PE, Gallagher EL, Miles, JR. 2003. Observations of bed level change Masselink G, Puleo JA. 2006. Swash-zone in a saturated surf zone. Journal of morphodynamics. Continental Shelf Geophysical Research 108: 3112. Research 26: 661-680. Schmidt WE, Woodward BT, Millikan KS, Masselink G, Hughes MG, Knight J. 2011. Guza RT, Raubenheimer B, Elgar S. 2003. A Coastal Processes and Geomorphology: GPS-Tracked Surf Zone Drifter. Journal of Second Edition. Hodder Education: London. Atmospheric and Oceanic Technology 20: McCarroll RJ, Brander RW, Turner IL, Power 1069-1075. HE, Mortlock TR. 2014. Lagrangian Scott TM, Russell PE, Masselink G, Wooler observations of circulation on an embayed A, Short AD. 2007. Beach Rescue Statistics beach with rip currents. Marine and their Relation to Nearshore Morphology Geology 355: 173-188. and Hazard: A Case Study for Southwest Morang A, Larson R, Gorman L. 1997. England. Journal of Coastal Research SI 50: Monitoring the Coastal Environment; Part 1: 1-6. Waves and Currents. Journal of Coastal Scott TM, Russell PE, Masselink G, Wooler Research 13: 111-133. A. 2008. High volume sediment transport and Puleo JA, Faries JWC, Davidson M, Hicks B. its implications for recreational beach risk. 2010. A Conductivity Sensor for Nearbed Proceedings of the 31st International Sediment Concentration Profiling. Journal of Conference on Coastal Engineering, ASCE, Oceanic and Atmospheric Technology 27: 4250-4262. 397-408. Senechal N, Coco G, Bryan KR, Holman RA. Puleo JA, Lanckriet T, Wang P. 2012. Near 2011a. Wave runup during extreme storm bed cross-shore velocity profiles, bed shear

British Society for Geomorphology Geomorphological Techniques, Chap. 3, Sec. 2.3 (2014) 13 Inch, K.W. conditions. Journal of Geophysical Research 116: C07032. Senechal N, Abadie S, Gallagher E, MacMahan J, Masselink G, Michallet H, Reniers A, Ruessink G, Russell P, Sous D, Turner I, Ardhuin F, Bonneton P, Bujan S, Capo S, Certain R, Pedreros R, Garlan T. 2011b. The ECORS-Truc Vert’08 nearshore field experiment: presentation of a three- dimensional morphologic system in a macro- tidal environment during consecutive extreme storm conditions. 61: 2073- 2098. Sheremet A, Guza RT, Elgar S, Herbers THC. 2002. Observations of nearshore infragravity waves: Seaward and shoreward propagating components. Journal of Geophysical Research 107: C8. Short AD, Brander RW. 1999. Regional Variations in Rip Density. Journal of Coastal Research 15: 813-822. Suhayda JN. 1974. Standing waves on beaches. Journal of Geophysical Research 79: 3065-3071. Thornton EB, Guza RT. 1982. Energy saturation and phase speeds measured on a natural beach. Journal of Geophysical Research 87: 9499-9508. Thornton EB, Guza RT. 1983. Transformation of Wave Height Distribution. Journal of Geophysical Research 88: 5925-5938. Van Dongeren A, Battjes J, Janssean T, Van Noorloos J, Steenhauer K, Steenbergen G, Reniers A. 2007. Shoaling and shoreline dissipation of low-frequency waves. Journal of Geophysical Research 112: C02011. Woodroffe CD. 2002. : form, process and evolution. Cambridge University Press: Cambridge.

British Society for Geomorphology Geomorphological Techniques, Chap. 3, Sec. 2.3 (2014)